The present invention generally relates to vortex shedding flowmeters, and more particularly to a vortex shedding flowmeter in which a bluff body having a simple construction is provided in a flow passage of a fluid for generating Karman vortex of the fluid which is to be measured.
Conventionally, a vortex shedding flowmeter has been reduced to practice wherein a bluff body is provided in a flow passage of a fluid and the current and flow rate of the fluid which is to be measured (hereinafter simply referred to as the measuring fluid) are detected by detecting a pressure change corresponding to the Karman vortex of fluid generated by the bluff body on the downstream side of the bluff body. In the conventional vortex shedding flowmeter of the type which uses electrostatic capacitance type detecting parts, a recess is provided on both side surfaces of the bluff body, and a fixed electrode plate and a diaphragm are provided in each recess. A minute gap is formed between the diaphragm and the fixed electrode plate, and the diaphragm is used as a movable electrode which is subjected to the pressure change of the measuring fluid. A fluid is sealed in a space between the fixed electrode plate and the diaphragm, and a pair of such spaces formed in the bluff body communicate with each other through a communicating passage. In this conventional vortex shedding flowmeter, the Karman vortex of the measuring fluid which passes both sides of the bluff body as the measuring fluid flows, is alternately generated on the downstream side of the bluff body along both side surfaces of the bluff body. The pair of diaphragms are displaced responsive to the pressure change of the measuring fluid caused by the Karman vortex, and the flow rate of the measuring fluid is measured by electrically detecting the change in the electrostatic capacitance which is formed between the diaphragm and the fixed electrode plate.
However, in order to improve the accuracy of the flow rate measurement in the conventional vortex shedding flowmeter, that is, to improve the accuracy with which the electrostatic capacitance is detected, it is necessary to accurately set the minute gap between the fixed electrode plate and the diaphragm which is used as the movable electrode to a predetermined distance. The recess into which the fixed electrode plate is fixed, must be formed with a high accuracy. Generally, however, the bluff body has a triangular or an approximately trapezoidal cross section, and it is extremely difficult to form the recesses on both side surfaces of the bluff body with a high accuracy because the side surfaces of the bluff body are mutually inclined by a predetermined angle. In addition, a similar difficulty is faced when welding the diaphragms so as to cover the respective recesses of the bluff body with a high accuracy. As a result, there are disadvantages in that the number of manufacturing processes are increased and the manufacturing cost becomes high. Especially since the troublesome process of forming the recesses on the side surfaces of each of the different bluff bodies must be performed for each of the vortex shedding flowmeters having flow passages of different diameters, the manufacturing cost of the vortex shedding flowmeter becomes high also from this point of view.
On the other hand, when a vibration occurs in the flow passage through which the measuring fluid flows, the fluid sealed in the detecting parts also vibrates, and there is a problem in that an erroneous detection may be performed. Hence, the present inventors have worked on the vibration in the flow passage, and as a result, the present inventors have found that the vibration in the flow passage rarely occurs in the longitudinal direction of the flow passage but occurs in most cases in a lateral direction which is perpendicular to the longitudinal direction of the flow passage. But in the conventional vortex shedding flowmeter, the communicating passage which communicates the pair of spaces in the detecting parts extends in the lateral direction which is perpendicular to the longitudinal direction of the flow passage. For this reason, when the fluid sealed in the pair of spaces and the communicating passage vibrates together with the flow passage, the sealed fluid is moved in mutually opposite directions with respect to the pair of spaces and the erroneous detection occurs. In the case where the amplification and sensitivity of a detecting circuit is reduced so as to reduce the effects of the vibration, the measuring sensitivity becomes reduced and there are disadvantages in that it becomes impossible to measure a minute flow rate and the range in which the flow rate can be measured becomes narrow.